The U.S. Patents listed above describe a device for dispensing fluid at a slow, uniform rate over a sustained period of time which has a simple, inexpensive structure, is easy to use, requires no regulation or adjustment by the person using the device, and which is well-safeguarded against tampering or inadvertent improper operation of the device.
The dispensing device described therein comprises a hose assembly adapted to be coupled to a conventional syringe, which hose assembly comprises a length of capillary tubing through which the fluid must pass while being delivered to a patient, and means for applying a uniform force to the plunger to provide fluid flow through the capillary tube at a slow, steady rate over a long period of time (e.g., less than 60 milliliters per hour).
As stated therein the rate of flow Q in cc/sec through the capillary tube can be estimated from Poiseuille's Law expressed in the equation: EQU Q=(Pr.sup.4)/(8ln)
where P is the pressure drop through the tube in dynes/cm.sup.2, r is the internal radius of the capillary tube in cm, l is the length of the capillary tube in cm, and n is the liquid viscosity in poise. By solving this equation it can be found that capillary tubes of a reasonable length suitable for restricting flow to rates in the range indicated under the influence of pressures of the range of pressures easily developed in a syringe (e.g., about 69,000 to 2,068,400 dynes/cm.sup.2) can have bores in the range of about 0.0025 to 0.038 cm. With current technology it is difficult to produce capillary tubing in this size range with bore diameters which deviate less than about 10% from a nominal diameter, however. Since the rate of flow through a tube is proportional to the fourth power of its diameter, such a deviation could cause a variation of about -34% to +46% in flow rate, which would be unacceptable for most medical uses. By only using long lengths of capillary tubing, however, (i.e., capillary tubes over 2 centimeter in length) much less variation in flow rates is found between different lengths of capillary tubing than is suggested above; perhaps because diameter variations tend to cancel each other along the length of the capillary tubes. With capillary tubing of polytetrafluoroethylene sold under the trade designation "Teflon" (which is preferred) having a nominal inside diameter in the range of 0.0025 to 0.019 cm, it has been found that lengths of the capillary tubing in excess of 2 cm. normally produce flow rate variations of less than 10%, which is acceptable for medical use of the device described herein.
Hose assemblies having capillary tubes of different nominal diameters which afford different rates of fluid flow (which rates, for example, may be indicated by color coding of the hose assemblies) can be used interchangeably in the device, and the inexpensive hose assemblies can be disposed of after use to insure sanitation for medical or other uses.
One problem presented by the use of such a hose assembly is that in many instances (such as for use in introducing fluids intraveniously) prior to activating the device, air must be purged from the hose assembly to preclude air being delivered to the patient. This purging is typically done by attaching the hose assembly to the syringe outside of the device, and manually activating the syringe until liquid has entirely filled the hose assembly; after which the syringe is placed in the device and the device is activated to deliver the liquid to the patient at the desired rate.
U.S. Pat. No. 4,430,079 teaches that for hose assemblies having small diameter capillary tubing (e.g., 0.0045 to 0.01 cm) adapted to alow only very slow rates of flow (e.g. 0.5 to 15 ml/hr) it is preferred to use a first embodiment of the hose assembly that allows the capillary tubing to be bypassed to afford rapid purging of air from the hose assembly with fluid from the syringe. The first hose assembly embodiment includes a metering assembly that provides a coupler between the hose assembly and the syringe. The metering assembly includes the capillary tube and parts moveable between a metering position at which fluid flowing through the metering assembly must pass through the capillary tubing, and a purging position at which fluid propelled by manual operation of the syringe can bypass the capillary tube and flow rapidly through the metering assembly to purge the hose assembly. The device has structure that insures that the metering assembly in this first embodiment of the hose assembly is in its metering position when the dispensing device is activated to preclude operation of the device with the metering assembly in its purge position. Support means included in a frame for the device supports the metering assembly and thereby a syringe coupled to the metering assembly. The support means is specially adapted so that it will engage and support the metering assembly only when it is in its metering position.
Also U.S. Pat. No. 4,430,079 teaches that for metering devices having larger diameter capillary tubing (e.g. over 0.010 cm) adapted to allow relatively larger rates of flow (e.g. over 15 ml/hr) one of several other embodiments of the hose assembly can be used that have no by pass, in which embodiments the capillary tubing may be positioned at the end of the hose assembly opposite the syringe. When the capillary tubing is positioned at the end of the hose assembly opposite the syringe almost all of the air passes through the capillary tube before the liquid reaches it, which, because of the extremely low viscosity of air compared to liquid, allows a fairly rapid purge rate (e.g. less than about 20 seconds). These other embodiments of the hose assembly include more conventional couplers at their ends adapted to engage the syringe, which couplers are adopted to be received in the support means of the device.
When either of the above described types of hose assemblies is made in a long length (e.g., over 45 centimeters and preferably in the range of 120 to 150 centimeters), however, it has a relatively large internal volume (e.g., 1/2 to 3/4 cubic centimeter) which is quite significant compared to the volume of the syringe from which the liquid is dispersed. Thus persons filling the syringe must add additional liquid to allow for the volume in the hose assembly (which will normally be still filled with the liquid when it is disgarded after use) while providing the intended delivered volume of liquid. This is both wasteful of the liquid, and causes an element of uncertainty in forecasting what internal volume the hose assembly will have.